1. Field
This is generally related to the manufacture of photovoltaic cells. More specifically, this disclosure is related to a method for improving the yield of photovoltaic cell manufacture.
2. Related Art
The negative environmental impact of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, photovoltaic power has been favored for its cleanness and wide availability.
A photovoltaic cell converts light into electricity using the photovoltaic effect. There are several basic photovoltaic cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Photovoltaic cells with a single p-n junction can be homojunction photovoltaic cells or heterojunction photovoltaic cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal bandgaps), the photovoltaic cell is called a homojunction photovoltaic cell. In contrast, a heterojunction photovoltaic cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.
In a photovoltaic cell, light is absorbed near the p-n junction generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a photovoltaic cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the photovoltaic cell is connected to an electrical circuit.
For homojunction photovoltaic cells, minority-carrier recombination at the cell surface due to the existence of dangling bonds can significantly reduce the photovoltaic cell efficiency; thus, a good surface passivation process is needed. In addition, the relatively thick, heavily doped emitter layer, which is formed by dopant diffusion, can drastically reduce the absorption of short wavelength light. Comparatively, heterojunction photovoltaic cells, such as Si heterojunction (SHJ) photovoltaic cells, are advantageous. FIG. 1 presents a diagram illustrating an exemplary SHJ photovoltaic cell (prior art). SHJ photovoltaic cell 100 includes front grid electrode 102, heavily doped amorphous-silicon (a-Si) emitter layer 104, intrinsic a-Si layer 106, crystalline-Si substrate 108, and back grid electrode 110. Arrows in FIG. 1 indicate incident sunlight. Because there is an inherent bandgap offset between a-Si layer 106 and crystalline-Si (c-Si) layer 108, a-Si layer 106 can be used to reduce the surface recombination velocity by creating a barrier for minority carriers. The a-Si layer 106 also passivates the surface of crystalline-Si layer 108 by repairing the existing Si dangling bonds. Moreover, the thickness of heavily doped a-Si emitter layer 104 can be much thinner compared to that of a homojunction photovoltaic cell. Thus, SHJ photovoltaic cells can provide a higher efficiency with higher open-circuit voltage (Voc) and larger short-circuit current (Jsc).
It has also been shown that tunneling-based heterojunction devices can provide excellent open-circuit voltage (Voc) from the combination of the field effect and surface passivation. To form such devices, ultra-thin quantum-tunneling barrier (QTB) layers are deposited on one or both sides of a crystalline Si base layer. However, the film quality of such ultra-thin QTB layers is very sensitive to environmental factors. Gaseous contaminants and moisture in the atmosphere can often cause degradation of the QTB layers and the corresponding junction structures, which leads to reduction of photovoltaic cell performance. Conventional cleanroom technologies have been widely deployed in integrated circuit (IC) fabrication. However, the fabrication of photovoltaic cells can be different from conventional IC fabrication. For example, the throughput of photovoltaic cell manufacturing facilities, in terms of the number of wafer processed, can often be much higher than that of IC manufacturing facilities. The footprint of a high-throughput photovoltaic cell manufacturing facilities can also be significantly larger than that of an IC manufacturing facility. The cost of implementing cleanroom capabilities throughout the entire photovoltaic cell fabrication facility can be prohibitively high.